Overheat-protecting device



7, 1967 MORIAKI KITAMI OVERHEAT-PROTECTING DEVICE Filed Aug. 20, 1965 INVENTOR.

, OVERHEAT-PRQTECTING DEVICE Moriaki Kitami, Akai Electric Co., Ltd., 883 3-chome, Kohjiya-cho, Ota-ku, Tokyo, Japan Filed Aug. 20, 1963, Ser. No. 303,223 4 Claims. (Cl. 317-41) This invention relates generally to a temperature responsive control device. It relates more specifically to an overheat-protecting device wherein the temperature of an element or equipment to be protected is sensed by a heat-sensitive element kept in heat-conductive relation therewith and when the sensed temperature has reached a predetermined value the current to a load circuit including said first element or equipment is shut off.

Various overheat or overcurrent protectors are commonly known. It can be, however, generally said that this kind of device acts only at a certain critical point to shut off the electric current.

It is however highly desirous to operate these overheat or overcurrent protectors within a wide tempeature range or along a temperature gradient at variable current increments in a specific dependence upon such temperature gradient.

The main object of this invention is to realize such novel overheat or overcurrent protecting devices which have the above mentioned wide temperature dependent operating characteristics.

' A further object of this invention is to provide protectors of the kind above referred to wherein an attempt to reset the circuit will not be successful if the real cause which has invited an overheat or overcurrent condition of the element or equipment to be protected within the specific wide operating range or on the temperature gradient should still remain.

In order to fulfill the above mentioned objects, the device according to this invention is characterized by the provision of a single core, twin-coil relay having two contact assemblies and a transistor amplifier circuit, one of said contact assemblies being connected with both first one of said coils and the load circuit including the element or equipment to be protected, the other contact assembly being connected with said transistor amplifier controlled by said heat-sensitive element so that the operating current of the relay is increased or decreased with decrease or increase of the temperature of the heat-sensitive element.

These and other objects and advantages as well as the essence of the invention will be apparent from the following description taken in connection with the accompanying drawing in which a preferred embodiment of the invention has been given by way of illustration only.

- Referring to the drawing:

FIG. 1 is a schematic representation of the preferred embodiment;

FIG. 2(A), (B) and (C) represents an explanatory representation showing three stages of operation of two contact assemblies employed in the invention;

FIG. 3 is a perspective view of an element or equipment to be protected from overheating or overcurrent, on the one hand, and a heat-sensitive element kept in heatconductive relation with said first element or equipment, on the other hand;

FIG. 4 is a diagram showing the temperature-current characteristic of a thermistor employed by way of example of the heat-sensitive element;

FIG, 5 is an example of an idealized temperaturecurrent characteristic of a semiconductor by way of example of the element to be protected; and

FIG. 6 is a diagram of idealized and practical curves of the maximum allowable current of an output circuit United States Patent 0 3,303,391 Patented Feb. 7, 1967 which is obtainable by the invention over a wide terriperatur'e range (by Way of example).

Now referring to the accompanying drawings, especially FIG. 1 thereof, a preferred embodiment of the invention will be described in detail. In FIG.- 1, 1 denotes a single core, twin-coil relay represented schematically in a block which includes a common core K, two coils L1 and L2 separately wound therearound and a pair of contact assemblies S1 and S2. Contact assembly S1 comprises two stationary contacts a and c and a movable contact b arranged therebetween. In the similar way, contact assembly S2 has two stationary contacts a and c and a movable contact b arranged therebetween. Contact b is electrically connected with one end of coil L1 and the opposite end thereof is connected through a lead 2 to a rectifier 3 which is connected in turn with one end of the secondary winding 4 of a power transformer T having a primary winding 5.

The primary winding is adapted to be energized from a current source 6, for instance, AC. 100 volts as shown. Stationary contact a is connected to any desired load circuit (not shown) through a lead 7 and an output terminal 8. The load circuit includes any desired element or equipment which should be protected from overheating or overcurrent by the arrangement according to the invention. As an example of such elements, a semiconductor or power transsistor 30 is shown in FIG. 3. It would be easily conceivable that such element or equipment may be a power transformer, thermal element, resistor, heater or the like. A condenser 9 is connected across the coil L1 so as to bypass possible A.C. component feed back from the load circuit. Stationary contact 0 is connected through a lead 10 including a push-off switch 11 to earth as shown. Stationary contacts 0 and a are merely backstops. Movable contact b' is connected through a lead 12 to the collector electrode of an amplifier or sensing transistor Tr. One end of relay coil L2 is connected through a lead 13 to a junction 14 in. the lead 2, while the opposite end of the coil is connected through a lead 15 to a junction 16 in the lead 12. A condenser 17 is connected across coil L2 so as to suppress a possible induced current in the secondary relay coil L2 on account of the induction coupling with the first relay coil L1. From a junction 18 in the lead 2, a lead 19 branches off and leads to earth through a thermistor 20, a junction 21 connected to the base electrode of transistor Tr and a variable resistor 22. The emitter electrode of the transistor is connected to earth as shown. From a further junction 23, a further lead 24 having a filter condenser 25 leads to earth which condenser is designed and arranged to bypass ripple current component coming from the rectifier 3.

It is important to note that the contact assemblies S1 and S2 are so designed and arranged that upon the energi- In the normal working conditionof the load circuit, a safe DC. current flows from earth through a load such as semi-conductor 30 shown in FIG. 3, lead 7, contacts a-b, coil L1, lead 2, rectifier 3 and the secondary winding 4 or transformer T. Current also flows from one end of the secondary winding 4, earth emitter and collector electrodes of transistor Tr, coil L2, lead 13, junction 14, lead 2, rectifier 3 to the opposite end of the said secondary winding. The coils L1 and L2 are wound in the assisting direction with each other so as to attract both movable contacts b and b towards the coils as, however, because of insufficient current, the two pairs of contacts a-b and a'-b' are kept in engagement as shown. It should be, however,

V or that of the semiconductor 30. acteristic curve similar to a hyperbolic of the relay may be kept in mind that under normal operating conditions with the variable resistor 22 properly set, the current flowing through coil L2 has a very small value, provided that the ambient air temperature is not exceptionally high.

If an overcurrent should flow through the load circuit, and thus through the first relay coil L1, the attractive force of the relay will be increased accordingly. Simultaneously, the power transistor 30, which is included in that load circuit will be heated up beyond the rated value and thus thermistor 20 fixedly attached on the transistor will be warmed up by the developed heat in the latter.

As commonly known, the thermistor element has such a conducting characteristic that with a temperature increase of the element the electric resistance thereof will be correspondingly decreased (see FIG. 4). Thus, under the above mentioned overcurrent condition, the bias resistance of the sensing transistor will considerably decrease so that the collector current will correspondingly increase which acts naturally to increase the attracting force of coil L2.

The increased attracting of the both coils L1 and L2 will act to break the both contact pairs a-b and a'b. However, in this case, the second contact pair will separate from each other and movable contact b will be brought into engagement with the opposite stationary contact c before the practical commencement of separation of the first contact pair a-b, as was already referred to. Should there be no such measure, the first con-tact pair will perform a vibration as commonly known to those skilled in'the art.

At the transient period, wherein the both contact pairs tab and ab are kept in engagement, the attractive force reaches the highest value by the energization of the both coils L1 and L2. But, this transient period is only instantaneous and the first coil L1 will be de-energized by the break of the first contact pair a-b is separated so as to break the overload current flowing therethrough. Before the occurrence of the break, contact pair b'c' of the second contact assembly S2 is closed and maintained in this position even upon the above described break of contacts a-b. Upon such a break, a substantial part of the current fed from the secondary winding 4 will fiow through earth, lead 10, the now closed contact pair bc', leads 12 and 15, coil L2, leads 13 and 2 and rectifier 3 again to the winding, on account of a considerable resistant value of sensing transistor Tr. By presetting the variable resistor 22 beforehand in the already described way, the current passing through the second coil L2 is selected to be enough large to close the lower contact pair bc and maintain this closed relation. By this action, power current flowing through the first coil L1 and delivered from terminal 8 to the lead circuit is naturally cut off.

When the temperature of the semiconductor or power transistor 30 to be protected from its overheating is increased under operating conditions for some reason or another, the current flowing through transistor Tr will correspondingly increase and thus the current through second coil L2 will increase by the same amount. This means that the relay current closing the contacts b'-c' increases with increase of the temperature of thermister 20 In this way, a charobtained, as shown by way of example in FIG. 6. Correspondingly, the relay actuation current may be so limited to describe a saw-tooth curve (when ideally considered) in function of the temperature of the semiconductor or the like element to be protected.

If a person should, upon an actuation of the relay by reason of overcurrent in the load circuit, depress the reset switch 11 or power switch 31 so as to feed power again to the load circuit, the relay will not be actuated since the circuit in still under overcurrent condition as above described. Only when the temperature of semiconductor 30 has decreased to a predetermined value, for instance, 60 C. as shown in FIG. 6, can the arrangement shown in FIG. -1 be reset. FIG. shows a rated temperature-current curve for semiconductor 30 to be protected (by way of example).

It is thus clear that the device explained above works within a wide temperature range with correspondingly variable relay-actuating current increments in dependence on the temperature of the element to be protected (by way of example).

Although the invention has been described in connection with a semiconductor as the element to be protected from overheat or overcurrent, it can be easily understood that any desired heat-sensitive element or equipment may be use-d instead of the said semiconductor.

On the other hand, the thermistor may be replaced by any desiredheat-sensitive resistant means having thermal characteristics other than that specifically described above.

Although for the purpose of explaining the invention a certain particular embodiment thereof has been dis-closed, obvious modifications will occur to a person skilled in the art, and it is not desired to be limited to the exact details shown and described.

The invention having thus been described, that which is believed to be new and for which protection by Letters Patent is desired, is: v

1. A temperature responsive control device for protection of a load against overheat conditions comprising, a power source, a reset switch, a single core, two-coil relay having two sets of contact assemblies with each of said sets having two positions, the first of said contact assemblies in a first position being electrically connectable to a load to be protected and being electrically connected to the first of said coils, the second of said contact assemblies in a first position being electrically disconnected from said reset switch and being connected to the second of said coils, a transistor amplifier electrically connected to said second coil and to a heat sensitive current-conductive element the resistance of which decreases with increase in temperature, said transistor amplifier in response to overheat condition sensed by said heat sensitive current-conductive element amplifying current from the power source to said second coil to energize said coil and thereby switch both of said contact assemblies to second positions in which said first contact assembly electrically disconnects from said load and said second contact assembly electrically connects to said reset switch While maintaining electrical connection to said second coil, whereby said contact assemblies are maintained in their second positions and the device cannot be reset by operation of the reset switch until the overheat condition terminates.

2. A temperature responsive control device as in claim 1, wherein said two contact assemblies are arranged so that the second contact assembly will switch to its second position before the first contact assembly will switch to its second position during an overheat condition.-

3. A temperature responsive control device as in claim 1, wherein said heat sensitive current-conductive element is connected to the "base circuit of said transistor amplifier.

4. A temperature responsive control device as in claim 1, wherein one end of said second coil is electrically connected to the collector electrode of said transistor amplifier and the other end of said second coil is electrically counected to the power source.

References Cited by the Examiner UNITED STATES PATENTS 2,802,059 8/ 1957 'Spack 317-132 X 2,808,471 10/1957 Poucel et al 317-41 X 2,859,402 11/ 1958 Schaeve 31741 X 2,885,604 5/1959 Starrinaki 31741 X 3,112,418 11/1963 Peras. 3,159,768 12/1964 Flanagan 317-41 MILTON o. HIRSHFIELD, Primary Examiner.

R. V. LUPO, Assistant Examiner. 

1. A TEMPERATURE RESPONSIVE CONTROL DEVICE FOR PROTECTION OF A LOAD AGAINST OVERHEAT CONDITIONS COMPRISING, A POWER SOURCE, A RESET SWITCH, A SINGLE CORE, TWO-COIL RELAY HAVING TWO SETS OF CONTACT ASSEMBLIES WITH EACH OF SAID SETS HAVING TWO POSITIONS, THE FIRST OF SAID CONTACT ASSEMBLIES IN A FIRST POSITION BEING ELECTRICALLY CONNECTABLE TO A LOAD TO BE PROTECTED AND BEING ELECTRICALLY CONNECTED TO THE FIRST OF SAID COILS, THE SECOND OF SAID CONTACT ASSEMBLIES IN A FIRST POSITION BEING ELECTRICALLY DISCONNECTED FROM SAID RESET SWITCH AND BEING CONNECTED TO THE SECOND OF SAID COILS, A TRANSISTOR AMPLIFIER ELECTRICALLY CONNECTED TO SAID SECOND COIL AND TO A HEAT SENSITIVE CURRENT-CONDUCTIVE ELEMENT THE RESISTANCE OF WHICH DECREASES WITH INCREASE IN TEMPERATURE, SAID TRANSISTOR AMPLIFIER IN RESPONSE TO OVER- 